for National Geographic News
Earth's first animals may have evolved in salty lakes, not oceans, a new study suggests.
Evidence for simple life on Earth stretches back billions of years, but the first multicelled animals didn't appear until a few hundred million years ago. (Explore a prehistoric time line.)
Many scientists have argued that these first, complex creatures arose in the oceans.
"It just seems like a fairly reasonable thing to assume, given the chemical and environmental stability of the oceans," said Martin Kennedy of the University of California, Riverside.
But a new study of one of the oldest known fossil beds has revealed abundant amounts of a mineral called smectite, which forms in salty, alkaline lakes—not seawater.
Southern China's Doushantuo formation, which dates back 600 million years, contains well-preserved fossil embryos of some of the earliest known animals, including sponges, jellyfish-like creatures, and early forms of a group of extinct corals.
At the time those animals lived, fungi and primitive plants had already colonized the land and were gradually increasing the levels of oxygen in the atmosphere.
Lakes absorb atmospheric oxygen more readily than the ocean, so early lakes may have become the first hospitable environments for animals, said Kennedy, lead author of the study, appearing in the current online issue of the journal Proceedings of the National Academy of Sciences.
"It would have taken the oceans much more time to have the same oxygen concentration that this lake had during this time," Kennedy added.
Other scientists have dismissed lakes as suitable homes for the first animals, on the grounds that such water bodies are often short-lived. Lakes typically last just a few thousand years, not long enough for life to evolve.
The lake at Doushantuo, however, lasted for tens of millions of years, and its longevity may have helped animal life gain a foothold, Kennedy said.
The new study is "incredibly interesting," said geobiologist Roger Summons of the Massachusetts Institute of Technology, who was not involved in the work.
Still, there could be other explanations for why older animal fossils haven't been found in marine sediments, he said.
For example, some environmental change may have occurred at Doushantuo 600 million years ago that allowed fossils to be preserved there in a way that wasn't possible in the oceans.
"This may be a preservational phenomenon as opposed to an evolutionary phenomenon," Summons said.
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